INCREASED CAPILLARITY IN LEG MUSCLE OF FINCHES LIVING AT ALTITUDE

An increased ratio of muscle capillary to fiber number (capillary/fibe r number) at altitude has been found in only a few investigations. The highly aerobic pectoralis muscle of finches living at 4,000-m altitud e (Leucosticte arctoa; A) was recently shown to have a larger capillar y/fiber number and greater contribution of tortuosity and branching to total capillary length than sea-level finches (Carpodacus mexicanus; SL) of the same subfamily (O. Mathieu-Costello, P. J. Agey, L. Wu, J. M. Szewczak, and R. E. MacMillen. Respir. Physiol. 111: 189-199, 1998) . To evaluate the role of muscle aerobic capacity on this trait, we ex amined the less-aerobic leg muscle (deep portion of anterior thigh) in the same birds. We found that, similar to pectoralis, the leg muscle in A finches had a greater capillary/fiber number (1.42 +/- 0.06) than that in SL finches (0.77 +/- 0.05; P < 0.01), but capillary tortuosit y and branching were not different. As also found in pectoralis, the r esulting larger capillary/fiber surface in A finches was proportional to a greater mitochondrial volume per micrometer of fiber length compa red with that in SL finches. These observations, in conjunction with a trend to a greater (rather than smaller) fiber cross-sectional area i n A than in SL finches (A: 484 +/- 42, SL: 390 +/- 26 mu m(2), both va lues at 2.5-mu m sarcomere length; P = 0.093), support the notion that chronic hypoxia is also a condition in which capillary-to-fiber struc ture is organized to match the size of the muscle capillary-to-fiber i nterface to fiber mitochondrial volume rather than to minimize interca pillary O-2 diffusion distances.